Sensitivity Analysis of Hole Cleaning Parameters in Directional Wells
- Jeff Li (BJ Services & Co.) | Scott Walker (BJ Services & Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- December 2001
- Document Type
- Journal Paper
- 356 - 363
- 2001. Society of Petroleum Engineers
- 4.2 Pipelines, Flowlines and Risers, 1.6.6 Directional Drilling, 1.7.1 Underbalanced Drilling, 1.11 Drilling Fluids and Materials, 1.10 Drilling Equipment, 2.4.3 Sand/Solids Control, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 3 Production and Well Operations, 1.6 Drilling Operations, 1.7.7 Cuttings Transport, 4.3.4 Scale, 3.2.5 Produced Sand / Solids Management and Control, 1.5 Drill Bits, 5.3.2 Multiphase Flow
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In this study, 600 tests were conducted and a new computer program was developed for the prediction of cuttings transport in the multiphase system(gas+liquid+cuttings). The sensitivity of cuttings bed height with respect to liquid/gas volume flow rate ratio, in-situ liquid velocity, rate of penetration(ROP), inclination angle, and circulation fluid properties was conducted. The hole-cleaning time with both circulation mode and wiper trip is also discussed. The results from the sensitivity analysis presented in this paper indicate, first, that the fraction of the circulation liquid has a significant impact on the cuttings transport in underbalanced drilling with gasified fluids;second, that among different variables, the in-situ liquid velocity is the most important variable for cuttings transport; and, finally, the hole-cleaning time decreases nonlinearly with increasing fluid circulation rate.
Cuttings transport has a major impact on the economics of the drilling process. Problems that may be caused by poor hole cleaning include stuck pipe, reduced weight on bit leading to reduced ROP, transient hole blockage leading to lost circulation conditions, excessive drill pipe wear, extra cost for special mud additives, and wasted time by wiper-trip maneuvers.
Sand production is a common problem faced by many of the oil producers, especially in the heavy oil industry. Several cleanout options have been developed over the years, employing a number of different technical approaches. One of most common options is running in with coiled tubing and circulating the sand out with a liquid or gasified liquid. An important consideration in designing cleanout operations is the proper selection of the pump rate and circulation fluid. These parameters should be optimized in order to transport the sand to surface with the least cost. High circulation rates can cause higher costs and other operational problems.
Over the past two decades, considerable effort has been expended into the research of cuttings transport in deviated wells with a single-phase flow. A large number of experiments have been conducted to study the effects of various parameters on cuttings-bed formation. A brief literature review was given by TUDRP.1-2 Experimental research on cuttings transport in inclined wells has been conducted using flow loops at the U. of Tulsa,3-6 the U. of Heriot-Watt,7 Mobil R&D Corp., M-I Drilling Fluid Co., BP Research Center, Inst. Francais du Petrole, and Petrobras.8-12
Several hole-cleaning correlations/models have been developed.4-6,13 Larsen,4 Jalukar,5 and Bassal6 conducted a total of 2,076 tests using the TUDRP's 5-in. and 8-in. flow loops. Based on the test results, they developed correlations to predict the critical velocity and cuttings-bed height. The correlations provide a means of analyzing cuttings transport as a function of operating conditions (flow rate, penetration rate, and rotation speed), mud properties (density, rheology), well configuration(angle, hole size, and pipe size), and cuttings size. Adari et. al.13 developed empirical models to correlate the cuttings-bed height and the hole-cleaning time to drilling fluid properties and flow rate for the wellbore near the horizontal position.
Similar to the models developed in the mining industry,14-16 layer mechanistic models to predict the bed height and pressure drop. However, such models are not able to model cuttings transport at medium inclination angles (35-55°) due to the chaotic motion of the cuttings.1 Furthermore, there are various unknown parameters, such as friction coefficients and a turbulent diffusion coefficient, involved in the models. These coefficients have to be determined based on comprehensive test results. There is still a gap between the model development and field application.21
Most of the previous cuttings transport studies in the drilling industry mainly focused on finding the minimum critical velocity and bed height or total cuttings concentration in the wellbore annulus for conventional rotary drilling with mud fluids. Recently, Rodriguez22conducted solids-transport tests with gasified fluids in near-horizontal wellbores. Based on the concept of critical velocity, he developed correlations to predict the minimum air and water flow rates required in order to avoid the formation of a stationary solids bed. However, this concept still lacks information related to cuttings transport with two-phase fluids and the prediction of the hole-cleaning time. In fact, there are different transport mechanisms during the bed erosion (ROP=0) process and during cuttings transport while drilling (ROP>0).21
In this study, a comprehensive experimental test of cuttings transport with two-phase fluids was conducted. The effect of liquid/gas volume flow rate ratio, in-situ liquid velocity, ROP, inclination angle, and circulation fluid properties on cuttings transport was investigated. However, this paper's main focus is on angles close to horizontal. Based on the test results, a computer program was developed and the sensitivity analysis of cuttings transport parameters, such as cuttings-bed height and hole-cleaning time, was conducted.
The test apparatus (Fig. 1) was designed and constructed in accordance with the following requirements.
Annular-flow steady-state conditions must prevail in every test case.
The apparatus must allow the selection of the most important drilling variables (gas and liquid flow rates, well inclination, annular geometry configuration, cuttings concentration, and drill pipe size/eccentricity, etc.).
The flow loop, shown in Fig. 1, was used throughout this project. The loop consists of a 20-ft transparent Lexan pipe with a 5-in. inner diameter simulating the openhole and a 2 3/8-in. steel inner pipe to simulate a drillpipe. The inner pipe is positioned on the bottom of Lexan pipe to simulate the worst case for the cuttings transport (eccentricity=100%).
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